In the unicellular green alga Selenastrum minutum, phosphoenolpyruvate carboxylase (PEPC) exists as two classes of isoforms with distinct kinetic properties. PEPC1 is a 400 kD homotetramer whereas PEPC2, PEPC3 and PEPC4 are large (> 1000 kD) protein complexes containing the 102 kD PEPC1 catalytic subunit. Association of the PEPC catalytic subunit with unrelated polypeptides appears to be responsible for the difference in kinetic properties between the tetrameric and protein complex PEPCs. PEPC2 constitutes the major form of extractable PEPC activity in S. minutum. When this protein complex is purified in the presence of metalloprotease inhibitors, a polypeptide with an approximate molecular mass of 130 kD (p130), is one of the major polypeptides present in the complex with the PEPC catalytic subunit. A polyclonal antiserum was generated against p130. Immunoblot analysis showed that p130 and the PEPC catalytic subunit copurify exactly during all four chromatographic steps of the purification, suggesting a tight interaction between p130 and PEPC in the high molecular mass PEPC isoforms. Further analysis revealed that p130 is phosphorylated in vitro by protein kinase A as well as an endogenous protein kinase from S. minutum. Work is currently in progress to identify p130 using mass spectrometry analysis of peptide mass maps and sequencing.